element_textbox_highlight() for strip text

[z3tt]

Hi Claus, again many many thanks for this package, it is one of the extension packages I use on a daily basis and teach to everyone interested in learning how to use ggplot.

During the workshops (and also recently on Twitter) people keep asking how to change the style of one or more strip texts. You have provided a solution to this as an reply to this SO question and I wonder if you ever considered adding this functionality to the package to make it easier to use for ggplot users?

Cheers,
Cédric